This invention relats to clutch servos for vehicle cable-operated clutch systems of the kind comprising a housing in which works an input member to which an input force from a pedal is applied by a cable assembly comprising a flexible inner cable and a flexible outer sheath, an output member for connection to the clutch, through which an output force acts, a movable wall for augmenting the output force, the movable wall being operated by pressurisation of a fluid pressure chamber, and valve means for controlling pressurisation of the chamber in response to the output force.
Clutch servos of the kind set forth may be vacuum-operated. In an operative position, both sides of the movable wall are connected to a vacuum source, and in operation the valve means admits air to the fluid pressure chamber to operate the movable wall. In a vehicle, the vacuum source for clutch operation tends to be uncertain, so it is necessary for the volume of the fluid pressure chamber to be kept to a minimum, in order to avoid, as far as possible, failure of the vacuum source. However, difficulties may arise if the system includes adjustment, whether automatic or manual, of the clutch cable assembly to compensate for clutch lining wear. If the adjustment, which increases the effective length of the inner cable, is performed between the pedal and the servo, the retracted position of the movable wall in the housing, and the volume of the fluid pressure chamber, alter accordingly. In fact, as the linings wear, the movable wall moves away from the pedal, so that the volume of the chamber is reduced, but then a larger volume is required when the clutch linings are new.